Is it possible the woman was with the other two people in that row? Nothing really explains that, but that would be a reason they might move someone.
Of course, if you paid extra to choose a specific seat, they definitely should have to pay you back if they move you out of that seat.
Company policy shouldn’t be to move someone from a specific seat they paid for unless it’s a true emergency type of situation. The comfort and convenience of another able-bodied passenger who did not think to pre-good that seat should not suffice.